Various methods for controlled expression of a recombinant nucleotide sequence of interest in a cell are known to the art. For example, No et al. (1996) Proc. Natl. Acad. Sci. USA 93:3346-3351, describe an inducible gene expression system utilizing a chimeric transactivator consisting of the ecdysone nuclear receptor fused to the VP16 transactivation domain. In the presence of inducer, this chimeric transactivator binds to recognition sequences upstream from a promoter and stimulates transcription of a nucleotide sequence of interest. In the absence of inducer, expression of the nucleotide sequence of interest is reduced and dependent on the basal level of transcription from the nucleotide sequence of interest promoter. Gossen et al. (1992) Proc. Natl. Acad. Sci. USA 89:5547-5551, describe a system for regulating expression of a nucleotide sequence of interest based on a chimeric protein, tTA, consisting of the TetR repressor protein fused with the VP16 transactivation domain. Similar to the ecdysone system, the DNA sequences specifying the TetR DNA binding site are inserted upstream of the gene promoter such that binding of the TetR-VP16 fusion protein stimulates transcription from the promoter and expression of the nucleotide sequence of interest. Other systems targeted to specific DNA binding sites proximal to a minimal promoter for targeted regulation of transcription utilizing the VP16 transactivation domain have also been developed, including GAL4-VP16 (Sadowski et al. (1988) Nature 335:563-564), LexA-VP16 (Brent et al. (1985) ET 712521864 US Cell 40:729-736), and Lacl-VP16 (Labow et al. (1990) Mol. Cell. Biol. 10:3342-3356). Other TetR-based systems are described in Deuschle et al. (1995) Mol. Cell. Biol. 15:1907-1914 and Yao et al. (1998) Hum. Gene Ther. 13:1939-1950.
Problems resulting from leaky expression related to the use of a minimal promoter have led to systems using fusions of the steroid-binding domains of the glucocorticoid or estrogen nuclear receptors (see, for example, Mattioni et al. (1994) Methods Cell Biol. 43:335-352; Louvion et al. (1993) Gene 131:129-134; Iida et al. (1996) J. Virol, 70: 6054-6059.